1. Field of the Invention
The invention relates to a system for controlling the combustion process for a gas turbine and, in particular, to a system for the active control of nitrogen oxide (NOx) emissions of the gas turbine and combustion pulsations. The system is particularly intended for gas turbine combustion systems having a plurality of pre-mix burners that lead to one single combustion chamber.
2. Brief Description of the Related Art
The NOx-emissions of combustions processes of gas turbines are subjected to environmental requirements that limit the levels of NOx emitted. In many countries, the limits for emissions levels are steadily lowered, presently to single-digit ppm-levels, and gas turbine manufacturers must be able to guarantee the required levels.
Gas turbines are required to operate at ever increasing efficiencies while producing lower levels of pollutants in their exhaust gases. Their efficiency can be increased, for example, by increasing the flame temperatures, however this also increases the levels of pollutants, in particular NOx, which is an exponential function of the flame temperature. One known method to control the NOx levels in a turbine's exhaust gas is to operate the burners with a maintained lean flame, i.e., at a lean fuel-to-air ratio, which lowers the flame temperature. However, a lean flame can lead to pulsations of the flame, or combustion pressure oscillations, which can reduce the lifetime of a burner. The formation of NOx and pulsations are influenced by several interconnected physical mechanisms such as the ambient atmospheric conditions, fuel composition, firing and flame temperature, combustion homogeneity, burner velocity, and others.
In essence, low NOx-emissions and pulsation levels, both of which need to be achieved in a gas turbine plant, are two counteracting physical processes. When a flame becomes leaner, then NOx-emissions decrease, however pulsations increase. Additionally, the more uniform the flame temperature resulting from multiple burners, the lower the NOx emissions and the greater the pulsations. Therefore, a trade-off between the two is necessary.
In gas turbine systems of the state of the art, pulsation and/or NOx level control is realized by various methods.
WO 2005/093326 discloses a gas turbine operating method for annularly arranged burners and combustion chambers that controls and reduces pressure pulsations by a system that individually actuates each burner and additionally monitors each burner using a sensor system.
U.S. Pat. No. 5,321,947 discloses a combustion system for a gas turbine engine having multiple swirler vanes arranged about a fuel injection nozzle. The combustion system is operated with a lean premix of fuel and with a cooled fuel injector in order to achieve reduced pollution. A low NOx level is maintained by a supply of combustible fuel into spaces between the swirler vanes, and combustion pressure oscillation are reduced by a continual supply of pilot fuel to the combustor at all operating conditions.
EP 1 621 811 discloses a method of operating a combustion system for a gas turbine engine having a plurality of burners. The fuel-to-air ratio for one or more burners is primarily controlled based on measured pressure oscillations or pulsations. In order to maintain the combustion close to the extinction limit, the fuel-to-air ratio is increased or decreased when pressure oscillations exceed a given lower or upper limit, respectively. Secondary to the control of pulsations, the fuel-to-air ratio is reduced when NOx levels are exceeded. In order to keep constant the total amount of fuel supplied to all the burners together, the fuel-to-air ratio for primary burners is increased while it is decreased for secondary burners.
EP 1 286 031 discloses a gas turbine control apparatus having a frequency analyzer of pressure oscillations in the combustion system and a control unit that controls, based on the result of the frequency analysis, the ratio of fuel and air supplied to the combustion chamber.
EP 1 331 448 discloses a fuel control system for a gas turbine engine having a plurality of burners and a plurality of combustion chambers, where each of the burners is associated with a separate, “can-type” combustion chamber. The system independently adjusts the fuel-to-air ratio of each individual combustion chamber to control the NOx level and dynamic pressure in each combustion chamber based on measurement of the NOx level in the exhaust, pressure oscillations in each combustion chamber, and fuel-to-air ratio variation among the burners. The adjustment is repeated until these variables are maintained within given ranges. The system is specifically designed for a gas turbine with multiple combustion chambers.
EP 1 283 339 discloses a method of remote monitoring of a gas turbine engine that operates at low NOX emissions. The method is intended to prevent damage due to resonances with turbine combustion oscillations and to provide low-cost communication of data to a remote monitoring center. It includes the measurement of oscillations data separated into frequency spectrum and peak value data. If abnormal peak values occur, then the frequency spectrum is observed. Before combustion oscillations develop to a critical level, where damage may occur, instructions are sent to operate the engine in a low-load operation mode. The monitoring is communicated by low-cost processes such as an ISDN-based line switching method.
EP 0 529 900 discloses a gas turbine apparatus having multiple burners and an annular combustion chamber and a controller to adjust the flow of fuel and/or air independently to each individual burner in view of controlling burner stability and NOx emissions. The individual adjustment is performed according to one or two sensed quantities relating to a combustion characteristic or performance quantity of each burner. Each quantity is measured separately, whereupon separate correction signals are determined for each. These are in turn added in order to generate a final control signal to a fuel and/or air valve.